1. Field of the Invention
This invention relates to a method for manufacturing an optical filter having a laminate comprising a plurality of films that are different in the refractive index stepwise.
2. Description of the Related Art
A method in which a plurality of lights that are different in wavelength are transmitted in an optical fiber has been employed to transmit a lot of data in the field of optical communication. This method depends on the property of the light that lights having different wavelengths do not interfere each other and behave independently. In the communication technology of this type, an optical filter for separating a laser light into laser lights of individual wavelengths is used on the input side and on the output side of an optical fiber.
It is required for an optical filter of this type to separate the wavelength of the light into finely divided bands. The refraction index gradient type filter is used as an optical filter of this type. The refraction index of the refraction index gradient type optical filter changes successively depending on the depth of laminate thickness of the film, and the light reflection of high order wavelength lights is prevented with respect to the transmission wavelength light by using an refractive index gradient type optical filter.
A refraction index gradient type optical filter is formed by means of TSH technique, helicon beam sputtering technique, or ion beam sputtering technique. For example, a plurality of targets having different refractive indexes such as TiO2 target and SiO2 target are subjected to ion beam sputtering by use of separate ion guns in a vacuum vessel, and the sputtering yield is controlled individually to form a film on a glass substrate. Thereby, a film having a laminate structure in which the refractive index changes depending on the depth of laminate thickness is formed on a glass substrate.
However, in the case of the conventional method in which two targets and two ion guns are used, the mix ratio of two types of material should be controlled precisely to obtain a film having a desired refractive index, and it is required to control the sputtering ratio with continuous accurate monitoring of the refractive index of the film. Also in the case of the optical filter of this type, it is required to change the refractive index stepwise for every film thickness while confirming that the monitored optical film thickness of a layer is identical with the design value. In this case, it is required that the refractive index difference between adjacent films should be small and that the boundary between adjacent films should be defined clearly. Therefore, it is required that the sputtering ratio of two materials should be controlled precisely from the initial stage of film forming of each film, and such a requirement is not satisfied practically.
Furthermore, another method for changing the refractive index stepwise in which a plurality of materials having different refractive indexes, for example, TiO2, Ta2O5, ZrO2, and Sio2, are combined has been used conventionally. However, the selectable range of the refractive index is limited, and particularly in the case where it is required to minimize the refractive index difference at the boundary plane between adjacent films, the selectable range is limited extremely.
Furthermore, in addition to the selectivity of the refractive index, the boundary between laminated layers is not smooth and a film is not uniform, and such irregularities may cause a problem. In the case of an amorphous film, because deposited particles that have been deposited on a substrate are not rearranged on the substrate through the film growing process, the uniformity and smoothness at the boundary are poor due to generation of particles in the film, and such irregularities cause light scattering and deviation from the designed performance.
The present invention has been accomplished to solve the abovementioned problem, and it is the object of the present invention to provide an optical filter characterized in that the refractive index is uniform within a predetermined film thickness and the film thickness of a film having the uniform refractive index is controlled precisely, a laminate film having the refractive index that changes stepwise can be formed, the slight refractive index difference between adjacent layers and the wide range refractive index selection are not limited, the boundary plane between films is smooth, and the refractive index difference is discrete, and to provide a method for manufacturing this type of optical filter.
A method for manufacturing an optical filter of the present invention is characterized in that a plurality of target materials containing a plurality of metal elements or metal oxides of the metal elements are disposed in a chamber of a sputtering apparatus, a mix ratio and/or types of the metal elements are differentiated between the target materials, the target materials are selected one by one successively to form a laminate film comprising a plurality of layers having refractive indexes that change stepwise at a boundary of an interface plane between adjacent films on a substrate.
The target material is preferably fine particle mixture of two or more metal oxides, and in this case it is preferable that the fine particle mixture contains materials having the refractive indexes that are different from each other discretely. For example, the combination of Ta2O5 and SiO2 or combination of TiO2 and SiO2 is most preferably used in view of the stress and refractive index of a film.
Furthermore, it is preferable that the target material is alloy that contains two or more metal elements and oxides of these elements to thereby balance between the absorptance and high refractive index easily. Examples of such material include, for example, alloy oxide of Alxe2x80x94Si or alloy oxide of Pbxe2x80x94Si.
It is preferable that an inert gas ion beam is used as sputtering particles ejected against the target material in the manufacturing method of the present invention.
In a method for manufacturing an optical filter of the present invention, a particle mixture of two or more types of metal oxides or a particle mixture of alloy oxides is used, and a plurality of target materials having respective different refractive indexes are used selectively to thereby laminate films having uniform refractive indexes successively that match with the designed structure while controlling the optical film thickness precisely.